The present invention relates generally to microsurgical instruments and more specifically to laser microsurgical instruments for use in cutting thin tissues or membranes. Prior to the present invention, thin tissue such as vitreous membranes were generally cut with a mechanical cutter or surgical knife. In the case of posterior segment surgery of the eye, a mechanically or pneumatically driven microscissors generally is used. However, vitreous surgery of tissues tightly adhered or adjacent to the retinal surface is very exacting because of the possibility of retina injury. As a result, a need exists for an alternative to these mechanical systems.
Midinfrared lasers long have been used for ablation of biological tissue because they generally emit radiation with a wavelength near the several water absorption peaks, resulting in an extremely short tissue penetration depth, generally between 1 micron (.mu.m) and 500 .mu.m. Examples of such lasers include the hydrogen fluoride laser, the erbium:yttrium aluminum garnet (Er:YAG) laser, the holmium:yttrium aluminum garnet (Ho:YAG) laser, the Raman-shifted neodymium:yttrium aluminum garnet (Nd:YAG) laser and the CO.sub.2 laser lasing at a wavelength of 10.6 .mu.m. However, the CO.sub.2 laser is not as useful as some other midinfrared lasers as a tissue ablater because the absorption depth of water at the relevant CO.sub.2 laser wavelength of approximately 10.6 .mu.m is ten times greater than at the wavelength emitted by the Er:YAG laser.
The Nd:YAG laser has been shown to be useful for photodisruption in the anterior segment of the eye, but generally is considered unacceptable for use in the posterior segment of the eye because of possible cavitation, acoustic and shock-wave effects and insufficient light divergence at the retina. Er:YAG lasers have been shown in experimental vitrectomies to be effective at cutting vitreous membranes. However, in some cases, retina injury resulted even though the fiber optic tip was held more than 1 mm from the surface of the retina. This extremely large damage zone is the result of gaseous bubble formation at the fiber optic tip.
As discussed by Lin, et al. in their article High-speed Photography of Er:YAG Laser Ablation in Fluid: Implication for Laser Vitreous Surgery, Invest. Oph. & Visual Sciences, 31(12):2546-2550 (Dec. 1990), when the Er:YAG laser is pulsed, the laser output consists of one or several submicrosecond spikes separated by a few microseconds. The first spike heats the liquid at the fiber optic tip, forming a bubble of hot gas. Subsequent spikes propagate readily through the bubble until they strike the outer liquid boundary of the bubble, thereby expanding the size of the bubble and allowing the thermal and mechanical energy to be transmitted to and damage an area of tissue much larger than that to be treated or cut. This article suggests either using low energy (below 0.5 mJ) but inefficient laser pulses and/or a shielded fiber optic tip to control or limit the expansion of the bubble. While a shielded tip might be effective in reducing the surface area of the tissue exposed to the bubble, the shielded tip does not affect the dwell time of the bubble at the tissue surface, likely resulting in a smaller yet deeper area of tissue damage. Therefore, neither the use of low energy pulses nor the shielded tip disclosed by Lin, et al. provide a laser tissue ablater that cuts tissue efficiently while minimizing collateral tissue damage.
In their article Erbium-YAG Laser Surgery on Experimental Vitreous Membranes, Arch. Oph., Vol. 31, pages 424-28, (Mar. 1989), Margolis, et al., discuss the use of a shielded tip in combination with an Er:YAG laser. A pulse energy of 3.6 mJ and a pulse repetition rate of 2 Hz was found to give the best tissue cutting results. Higher pulse repetition rates resulted in hot jets of vitreous flow to the retina that the authors believed caused the observed retinal lesions. The authors did not attempt to cut tissue closer than 1000 .mu.m to the retinal surface despite their recognition that many ophthalmic surgical procedures require cutting membranes less than 500 .mu.m from the retina.
Similarly, in their article Holmium-YAG Laser Surgery on Experimental Vitreous Membranes, Arch. Oph., Vol. 109, pages 1605-09 (Nov. 1991), Borirakchanyavat, et al., discuss the use of a Ho:YAG laser to cut vitreous membranes. However, the authors found that the most efficient tissue cutting occurred at a pulse repetition rate of between 1 Hz and 2 Hz and a pulse energy of greater than 60 mJ. At this relatively high pulse energy, tissue cutting without retina damage was limited to distances of greater than 0.5 mm from the retina despite the use of a shielded tip. Furthermore, the Ho:YAG laser emits radiation at a wavelength of approximately 2.12 .mu.m. The tissue absorption length at this wavelength is approximately 430 .mu.m, far greater than the tissue absorption length of approximately 1 .mu.m of the radiation emitted by the Er:YAG laser. This relatively long tissue absorption length makes it difficult to irradiate thin membranes without the radiation penetrating deeper into and damaging surrounding tissue.
Accordingly, a need continues to exist for an efficient midinfrared laser tissue ablater that reduces the amount of collateral tissue damage while permitting the cutting of tissues tightly adhered or adjacent to the retinal surface.